1. Field of the Invention
The present invention relates to a technique of receiving a digital signal, and particularly to a technique of demodulating a digital signal for which an orthogonal frequency division multiplexing (OFDM) method is employed as a signal transmission method.
2. Description of the Related Art
A method referred to as an orthogonal frequency division multiplexing (OFDM) method is introduced as a method of transmitting a digital signal. The OFDM method is a method of assigning data to a plurality of carriers orthogonal to the frequency axis to transmit the data and in which modulation is conducted by IFFT (inverse FFT (Fast Fourier Transform)) and demodulation is conducted by FFT.
The OFDM method realizes a high efficiency of frequency use such that its application to ground wave digital broadcasting is widely discussed and is employed in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), which is a standard for ground wave digital broadcasting in Japan.
In ground wave digital broadcasting in Japan (ISDB-T), digital television broadcasting using the UHF band (13ch to 62ch) and digital radio broadcasting using the VHF band (7ch and 8ch) are implemented.
In digital television broadcasting, as shown in FIG. 1A, a transmission band of one channel of 6 MHz is divided into thirteen segments. Twelve segments are used for hi-vision broadcasting whose main target is indoor television receivers with fixed reception, and the remaining segment is used for broadcasting whose main target is movable bodies such as mobile terminals and the like (called “one-segment broadcasting”). Depending on channels, broadcasting is performed using all thirteen segments for fixed terminals.
In digital radio broadcasting, as shown in FIG. 1B, the transmission band of one channel of 6 MHz is divided into eight segments, and one or three of these segments are used for broadcasting.
In the above situation, two types of receivers are possible as reception terminals for mobile reception of ground wave digital broadcasting, i.e., a 1-segment receiver that can only perform 1-segment reception and a 1-segment/3-segment compatible receiver that can perform 3-segment reception in addition to 1-segment reception (referred to as “⅓-segment receiver” hereinafter).
FIG. 2 shows a normal configuration of a receiver of digital broadcasting in accordance with ISDB-T.
In FIG. 2, a high-frequency signal that has been digitally modulated using the OFDM method is received by an antenna 101. The received high-frequency signal is input into a tuner 102. The tuner 102 selects a channel to be used for the reception, converts the high-frequency signal on the selected channel into an intermediate frequency signal (IF signal), and outputs the intermediate frequency signal to an A/D (analog/digital) converter 103. The A/D converter 103 converts the intermediate frequency signal, which is an analog signal, into a digital signal.
The signal output from the A/D converter 103 is input into a quadrature demodulator 104 in order to be converted into a complex baseband signal. The complex baseband signal is converted from a time domain signal into a frequency domain signal using fast Fourier transform (FFT) performed by an FFT circuit 105, and respective carrier data is obtained.
Thereafter, the carrier data is input into an equalization process unit 106, then an interpolation process of a pilot signal and an equalization process of the carrier data are executed and the carrier data is output as demodulated data. The demodulated data output from the equalization process unit 106 is input into an error correction unit 107 where an error correction process is executed. Thereafter, the demodulated data is output in the form of a TS (Transport Stream).
A part of the carrier data output from the FFT circuit 105 is input into a TMCC (Transmission and Multiplexing Configuration Control) error correction unit 108 where an error correction regarding TMCC information is executed. The TMCC information superposed on a digital signal of the OFDM method is control information in which transmission parameters used in each carrier are specified, including parameters such as modulation method and interleave length in a time interleave. The TMCC information on which the effort correction was executed is used in respective units such as the error correction unit 107 of the receiver of FIG. 2.
Next, FIG. 3 is explained. FIG. 3 explains in detail the error correction process executed by the error correction unit 107 of FIG. 2.
As shown in FIG. 3, in the error correction unit 107, a demapping process 113, a viterbi demodulation process 115, a Reed-Solomon demodulation process 117, and various deinterleaving processes are executed. Specifically, a frequency deinterleaving process 111 that is a deinterleaving process of the frequency axis direction, and a time deinterleaving process 112 that is a deinterleaving process of the time axis direction are first executed on the demodulated data output from the equalization process unit 106. Thereafter, the demapping process 113 is executed. Then, a bit deinterleaving process 114 that is a deinterleaving process in units of bits is executed on the data on which the demapping process 113 has been executed; thereafter, the viterbi demodulation process 115 is executed. Further, the data on which the viterbi demodulation process 115 has been executed is converted from serial to parallel in units of bytes. Thereafter, a byte deinterleaving process 116, which is a deinterleaving process in units of bytes, is executed. Then, the Reed-Solomon demodulation process 117 is executed.
Japanese Patent Application Publication No. 2003-188848 discloses a technique in which a time interleaving process is executed on compressed data created by bit compression of demapped data. Thereafter, bit expansion is performed on compressed data that has been subject to a time deinterleaving process in order to reduce memory capacity in an OFDM receiver.
Japanese Patent Application Publication No. 2003-78839 discloses a technique in which, when a voice broadcasting signal is received, the TMCC information is first obtained by handling the band of a frequency conversion unit output signal as the boundary frequency of the pass band of an IF filter that is used in television broadcast reception. Then, the frequency conversion unit and a filter for voice are controlled such that a Fourier transform is performed over a bandwidth of one segment or of three segments in accordance with the broadcasting method specified in the TMCC information; thereby, reception of both television broadcasting and voice broadcasting is realized in a digital broadcasting receiver that receives a signal demodulated with the OFDM method.
Japanese Patent Application Publication No. 2002-77095 discloses a technique in which I and Q data obtained as a result of quadrature demodulation on a received signal is stored in a storage unit and prescribed data is read from the storage unit to provide it to an FFT circuit. At the same time, prescribed data is read from the storage unit to provide it to a synchronous demodulation circuit in order to integrate the input buffer of the FFT circuit and a delay unit of the synchronous demodulation circuit.
Japanese Patent Application Publication No. 2003-101505 discloses a technique in which pre-demap data specifying the relationship between a reference point on a constellation and the amount of shift from the reference point in accordance with the modulation method of a signal is created from the output of an FFT unit, and in which a time deinterleaving process is executed on the pre-mapping data in which the shift amount has been corrected on the basis of the reliability information of a received signal. These two functions are performed so that the memory capacity required for the time deinterleaving process is reduced in a digital broadcasting receiver that receives a signal demodulated with the OFDM method.
Japanese Patent Application Publication No. 2004-282625 discloses a technique for reducing the memory capacity required for a frequency deinterleaving process by performing demapping and bit compression before executing the frequency deinterleaving process in an OFDM receiver.
In the error correction unit 107 of the ISDB-T receiver shown in FIG. 2, various interleaving processes are executed. As shown in FIG. 3, memory 111a, memory 112a, memory 114a, and memory 116a are included; the deinterleaving processes thus generally require memory capacity for temporarily storing data. Accordingly, the memory prepared for the deinterleaving processes occupies a very large proportion of the entire configuration of the ISDB-T receiver.
Also, in the ⅓-segment receiver, memory capacity for the deinterleaving processes that is three times larger than that in the 1-segment receiver is required. In other words, a large memory capacity has to be provided in the ⅓ segment receiver in order to receive 3-segment broadcasting, which only involves partial operation of the receiver.